In the below-listed Non-patent Document 1, a known method of reproducing a weak signal is disclosed. In this known method, the weak signal is split into N signals, independent noises are added to these signals, the resultant signals are input to nonlinear elements, and the outputs of the N nonlinear elements are combined, whereby the weak signal is reproduced. This method reproduces a weak signal through unitization of a phenomenon that, when a weak signal buried in noise is input to a threshold response element having a threshold characteristic, the probability of the weak signal exceeding a threshold increases during a period during which the weak signal is present. The phenomenon that the output of the threshold response element has a higher probability of exceeding the threshold during the period during which the weak signal is present is called “stochastic resonance.”
In the case where a single nonlinear element is used, for reproduction of a weak signal, it is important that the probability of the input signal exceeding a threshold increases during the period during which the weak signal is present and decreases during the period during which the weak signal is absent. Therefore, the detection accuracy of the signal increases when the relation among the threshold, the level of the weak signal, and the noise level is proper. Also, there exists an optimal noise level at which the detection accuracy is maximized.
The method of Non-patent Document 1 increases the detection accuracy of the weak signal irrespective of noise level by combining the outputs of a large number of nonlinear elements. Namely, when the noise level increases, the output of each threshold response element has a high probability of exceeding a threshold during both the period during which the weak signal is present and the period during which the weak signal is absent. In the case where the input signal exceeds the threshold due to the noise level, since the N noises to be superimposed are independent of one another, the phases at which the input signal exceeds the threshold in the N threshold response elements become random. Meanwhile, in the case where the input signal exceeds the threshold due to the presence of the weak signal, since the weak signals contained the N input signals are in phase, the outputs of the threshold response elements are also in phase. As a result, when the outputs of the N threshold response elements are combined into a single signal, the output level increases during the period during which the weak signal is present and decreases during the period during which the weak signal is absent. On the basis of such a principle, the method of Non-patent Document 1 improves the reproduction accuracy of a weak signal irrespective of noise level.
As disclosed in Patent Document 1, there has been known an apparatus which realizes the method which is disclosed in Non-patent Document 1 and in which N input signals obtained by superimposing independent noises on a weak signal are input to nonlinear elements and the outputs of the nonlinear elements are combined into a single signal. In the known apparatus, field effect transistors are used as nonlinear elements and are operated in a sub-threshold region.
Patent Document 2 discloses a circuit which improves the S/N ratio by utilizing a stochastic resonance phenomenon using a single nonlinear element and also discloses the relation between the nonlinear characteristic of the nonlinear element and the S/N ratio.